Boric-Acid-Functional Lanthanide Metal–Organic Frameworks for Selective Ratiometric Fluorescence Detection of Fluoride Ions

Here, we report that boric acid is used to tune the optical properties of lanthanide metal–organic frameworks (LMOFs) for dual-fluorescence emission and improves the selectivity of LMOFs for the determination of F^– ions. The LMOFs are prepared with 5-boronoisophthalic acid (5-bop) and Eu³⁺ ions as the precursors. Emission mechanism study indicates that 5-bop is excited with UV photons to produce its triplet state, which then excites Eu³⁺ ions for their red emission. This is the general story of the antenna effect, but electron-deficient boric acid decreases the energy transfer efficiency from the triplet state of 5-bop to Eu³⁺ ions, so dual emission from both 5-bop and Eu³⁺ ions is efficiently excited at the single excitation of 275 nm. Moreover, boric acid is used to identify fluoride specifically as a free accessible site. The ratiometric fluorescent detection of F^– ions is validated with the dual emission at single excitation. The LMOFs are very monodisperse, so the determination of aqueous F^– ions is easily achieved with high selectivity and a low detection limit (2 μM). For the first time, we reveal that rational selection of functional ligands can improve the sensing efficiency of LMOFs through tuning their optical property and enhancing the selectivity toward targets.